Various types of eye tracking, or gaze tracking, systems for determining a direction of a person's gaze and what the person is looking at, are known in the art. The systems are used, by way of example, for ergonomic and medical research, diagnostics, and interfacing a person with a computer, or a computer generated artificial environment.
Generally, the systems operate to determine a location for the person's pupil, and a gaze direction along which the person is looking, which is defined as a direction of a “gaze vector” that extends out from the eye along a line from a center of rotation of the eye through the center of the located pupil. A location of the eye in three-dimensional (3D) space is determined and used to determine coordinates of a region of space through which the gaze vector passes. The determined coordinates of the region, hereinafter referred to as an “origin” of the vector, locate the gaze vector in space. Given the direction and origin of the gaze vector, its intersection with a region or object in the person's environment is identified to determine what the person is looking at, which, presumably, is a point regard (POR) to which the person's attention is directed.
Hereinafter, a POR, is assumed to be coincident with an intersection of a person's direction of gaze and an object or region in the person's environment, and is used to refer to the intersection, object, and/or region. A gaze tracking system, hereinafter referred to as a “gaze tracker”, provides both a direction and an origin for a person's gaze vector, and optionally a POR, for the person.
Intrusive and non-intrusive methods and gaze trackers exist for determining a direction for a gaze vector. In some intrusive gaze trackers a person wears special contact lenses comprising induction micro-coils that move with the eye and pupil. A high frequency electromagnetic field is used to track orientation of the micro-coils and thereby the person's eyes and direction of gaze. In some intrusive gaze trackers, a person is fitted with electrodes that sense changes in orientation of a dipole electric field that the eye generates to determine direction of gaze.
Non-intrusive gaze trackers and tracking methods often image reflections, referred to as “Purkinje reflections”, of light from surfaces of different structures of the eye and process images of the reflections to determine their relative motion, and therefrom changes in direction of a person's gaze. The changes in gaze direction are referenced to a reference gaze direction to determine the person's gaze direction. First, second, third, and fourth Purkinje reflections refer respectively to reflections from the front surface of the cornea, from the back surface of the cornea, the front surface of the lens and the back surface of the lens.
For a given stationary source of light, reflections from the front surface of the cornea, the first Purkinje reflection, are strongest and are conventionally referred to as “glints”. Locations of images of glints are relatively independent of direction of gaze for moderate eye rotations (eye rotations up to about ±15°) and a fixed position of the head. Locations of images of glints are typically used to reference motion of images of features of the eye and/or of other Purkinje reflections to determine changes in a person's gaze direction.
In many non-intrusive gaze trackers, changes in location of an image of the pupil relative to an image of a glint are used to determine gaze direction. In some non-intrusive gaze trackers, reflections of light from the retina, which are not usually classified as a Purkinje reflections are used to image the pupil and track eye motion and gaze direction. The retina acts like a retro reflector and light that enters the pupil and is reflected by the retina exits the pupil along a direction that it entered the eye and backlights the pupil. The retinal backlighting of the pupil produces the familiar “bright eye”, or “red eye” effect, frequently seen in images of people's faces acquired with a flash. Bright eye pupil images of a person are acquired by a camera using light sources that illuminate the person's face from a direction substantially coincident with the camera optic axis. Locations of the bright eye pupil in the images are tracked relative to locations of glints in the images to determine the person's gaze direction. Bright eye pupil images are not produced by off axis light sources, and for off axis light sources, an imaged pupil appears dark. In many non-intrusive gaze trackers, locations of “dark pupil images” are compared to locations of images of glints to determine direction of gaze.
For many applications of a gaze tracker, a person's head is required to be stabilized relative to components of the gaze tracker so that it can provide acceptably accurate determinations of a direction and an origin for a gaze vector, and therefrom a POR for the person. For some gaze trackers, the person's head is stabilized by a static support, such as a chin rest often used in ophthalmic examinations, or a bite bar, to fix the head and eyes relative to components of the gaze trackers.
For applications such as interfacing a person with a virtual or augmented reality, it is advantageous for the person to be able to freely move his or her head and for these applications, a person typically wears a headgear, such as a helmet or goggles, that comprises gaze tracker components. The headgear holds the gaze tracker components in substantially fixed locations relative to the person's head and provides fixed, known, distances and orientations of the eye relative to the components. The known distances and orientations facilitate determining gaze vector directions and origins for the person relative to the headgear. Gaze vector directions and origins relative to the real world, a virtual or augmented reality, are determined from the gaze directions and origins relative to the headgear, and orientation of the headgear in the real world. Orientation of the headgear is determined using any of various optical, electromagnetic and/or mechanical position and orientation sensor systems.
Some gaze trackers provide directions and origins of gaze vectors and PORs for a person without recourse to a worn headgear. However, these gaze trackers generally operate for head positions restricted to a relatively small range of distances between about 50 cm and about 80 cm from the gaze trackers.